You really don't need to worry about the license file, because all that is handled automatically by our license servers. Just reformat your hard drive, then reinstall all your plugins (you can find the installers on the Manage Registered Plugins page), and then the first time you use one of your PA plugins, you'll be prompted to reauthorize. When you do, our servers will still be able to recognize that you’re using the same computer, so you can reauthorize it without wasting an authorization. If you'd like to check to be sure you're not wasting an authorization, you can go to the Manage Devices page, before you format and reauthorize your plugins, and then again afterward. If a new machine does pop up, that means something went wrong and the license server didn't recognize that it's the same computer. But don't worry, if that does happen, you can just click the "Deactivate" button next to the older machine name, so you're not wasting that authorization.
By the way, if you prefer, you can create a portable licenser with a standard flash drive. Then you can just use your plugins on any computer that has the flash drive plugged into it. Here's how:
Make sure your USB drive is in your computer. Then, open one of your plugins in ProTools (or any host) and click the key icon on the bottom right side of the plugin window. An authorization window will pop up. Under the text "Select the device you would like to activate" there is a pulldown menu. Click it and select your USB drive. Make sure the "Activate all my licenses" checkbox is checked. Then click "I'm Online" and enter your login info and click "Activate." Now your USB drive is authorized and can be used on any computer.